It is frequently necessary to sleeve an electrically conducting wire with an electrically insulating material, for example to prevent electrical coupling between adjacent wires within a cable, or to a conducting medium within which the wire is disposed. It is important that the insulation material performs adequately in the task, and one important figure of merit is the insulation resistance of the insulating material.
In subsea applications, insulated wires are used in the conductive medium of seawater, and the insulation material prevents electrical losses thereto, as well as fulfilling a number of other functions. Subsea cable insulation may degrade over time, with the insulation resistance eventually becoming unacceptably low. One cause of failure is seawater ingress into and through the insulation due to, for example, long term degradation of the insulating material, manufacturing faults or other means. Such a failure may result in short circuits between conductors and/or current flows from live conductors to earth. However, damage or other faults may lead to a reduction in insulation resistance.
In subsea hydrocarbon production systems, these types of failure can eventually lead to total loss of subsea electrical control and hence to an unplanned shutdown of production from one or more wells. Present solutions to the problem rely heavily on intervention to disconnect subsea equipment, cables and connectors and to replace with new items. There are a number of known devices, such as the arrangement described in GB2476152, that can be used to monitor the insulation resistance of a cable and to provide a warning to the user that the cable insulation material has degraded and a fault has occurred or is developing. Upon identification that the insulation has degraded or a fault has occurred, typical solutions rely heavily on intervention to disconnect subsea equipment, cables and connectors and to replace the removed equipment with new items. Such intervention is very inefficient, time consuming and expensive.
WO2010/136284 describes an arrangement for monitoring the insulation resistance of an ungrounded electrical network such as that found in electric and hybrid vehicles. It involves injecting a test voltage including an AC component and a DC component into the network. Appropriate monitoring allows measurements of the insulation resistance to be made.
A need exists for a method and apparatus that is capable of improving the insulation properties of cables, particularly subsea cables as used in a subsea electrical distribution system. Whilst as mentioned above WO2010/136284 describes an arrangement for monitoring insulation resistance (albeit in an application far removed from the present invention), it does not provide teaching regarding enhancement of or rejuvenation of the electrical insulating properties of a network or parts thereof.
US2010/0122453 and US2009/0133799 both describe arrangements intended for use in the rejuvenation of cables. However, neither relates to the rejuvenation of subsea systems or addresses the issues peculiar to subsea located equipment.